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Published byFelicity McCoy Modified over 9 years ago
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Photosynthesis – The Calvin Cycle
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Calvin Cycle Incorporates atmospheric CO 2 and uses ATP/NADPH from light reaction Named for Dr. Melvin Calvin He & other scientists worked out many of the steps in the 1940s Sometimes called “dark” reaction
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Overview Occurs in the stroma CO 2 enters the cycle and leaves as sugar Spends the energy of ATP and NADPH Glucose not produced - yield is: glyceraldehyde-3-phosphate (G3P) WHERE HAVE WE SEEN G3P BEFORE?
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One turn of the Calvin… Each turn of the Calvin cycle fixes 1C For net synthesis of one G3P molecule, cycle must occur 3X, fixing 3CO 2 To make one glucose molecule: 6 cycles and the fixation of 6CO 2
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Calvin Cycle has 3 Phases 1. Carbon Fixation Phase (Carboxylation) 2. Reduction 3. Regeneration of CO 2 acceptor (RuBP)
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1. Carbon Fixation 1CO 2 attaches to a 5C sugar ribulose 1,5 bisphosphate (RuBP) Catalyzed by (RuBisCO) ribulose-1,5-bisphosphate carboxylase/oxygenase
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1. Carbon Fixation 6C intermediate is unstable Immediately splits in half: forms 2 molecules of 3-phosphoglycerate
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2. Reduction 2 ATP needed for this step (per 1CO 2 ) Each 3-phosphoglycerate is phosphorylated forms 1,3-bisphosphoglycerate Pair of e - from NADPH reduces each 1,3- bisphosphoglycerate to: G3P Reduction of a carboxyl group to a carbonyl
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Crunch the Numbers… To produce one G3P net: start with 3CO 2 (3C) and 3RuBP (15C) After fixation/reduction: 6 molec of G3P (18C) One of these 6 G3P (3C) is a net gain of a carbohydrate Molec. can exit cycle to be used by plant cell Other 5 G3P (15C) must remain in the cycle to regenerate 3RuBP
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3. Regeneration of CO 2 The 5 G3P molecules are rearranged to form 3 RuBP molecules 3 molecules of ATP spent (one per RuBP) to complete the cycle and prepare for the next
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Crunch the Numbers…again Net synthesis of 1 G3P molecule, Calvin cycle consumes 9ATP and 6NAPDH “Costs” three ATP and two NADPH per CO 2
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Dehydration Land plants can easily dehydrate Stomata open to allow O 2 /CO 2 exchange Allows for evaporative loss of H 2 O Hot dry days – plants close stomata to conserve H 2 O PROBLEM!
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C 3 Plants C3 plants (most plants – rice, wheat, soy are examples) use RuBisCO and end product is G3P Stomata closed CO 2 levels drop (consumed by Calvin) O 2 levels rise (produced by light rxn) When O 2 / CO 2 ratio increases, RuBisCO can add O 2 to RuBP
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Photorespiration O 2 + RuBP yields 3C and 2C pieces (photorespiration) 2C piece exported from chloroplast, peroxisomes & mitochondria degrade to CO 2 Produces no ATP, no organic molecules Photorespiration decreases photosynthetic output
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WHY? EVOLUTION! Early Earth had little O 2, lots of CO 2 Alternative pathway negligible TODAY… Photorespiration can drain up to 50% of fixed carbon on a hot day Might evolution have come into play again?
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C 4 Plants Very common pathway – sugarcane, corn Mesophyll cells incorporate CO 2 into organic molec Phosphoenolpyruvate carboxylase adds CO 2 to phosphoenolpyruvate (PEP) to form OXALOACETATE. PEP Carboxylase has a high affinity for CO 2 – can fix C when RuBisCo can’t (i.e. when stomata are closed)
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Mesophyll cells pump 4C cmpds to bundle sheath cells BS cells strip a C (as CO 2 ) and return the 3C to mesophyll BS cells then use RuBisCO to start Calvin Cycle
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So… Mesophyll cells pump CO 2 into BS cells, so RuBisCO doesn’t need to utilize O 2. C 4 plants minimize photorespiration & promote sugar production Thrive in hot regions with intense sun
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CAM Plants Other plants have evolved another strategy to minimize photorespiration Succulents: Cacti, pineapples, several others CAM – Crassulacean Acid Metabolism Stomata open at night ONLY!
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Night: Fix CO 2 into a variety of organic acids in mesophyll Day: Light rxns supply ATP & NADPH to Calvin; CO 2 released from acids CAM Mechanism
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CAM & C 4 Add CO 2 to organic intermediates before entering Calvin In C4, carbon fixation and Calvin cycle PHYSICALLY (space) separated In CAM, carbon fixation and Calvin cycle are TEMPORALLY (time) separated
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